The present invention relates generally to a drill, and more particularly to a variable-speed drill. Still more particularly, the invention relates to a hand-held variable-speed drill.
Drills of this type have safety couplings which are constructed as slip couplings and which transmit the motor torque via radial pressure to the tool spindle or output shaft. This type of construction is acceptable in certain drill equipment, namely drill equipment (mostly used for do-it-yourself applications) wherein the torque acting upon the tool spindle is on the order of 20-30 cmkp and wherein the safety coupling is not subjected to particularly heavy-duty requirements with respect to the frequency with which it must operate and the accuracy of the disengaging moment.
However, drills which are used commercially, that is in the industry, in crafts and the like, are usually of heavier-duty construction and transmit to the tool spindle a torque that is considerably higher than the one mentioned above. The torque is usually on the order of 90 cmkp and if such a torque were to be transmitted through a radial-pressure slip coupling as in the aforementioned types of constructions, the coupling would have to be excessively large. This, however, would then result in a loss of the advantages of that type of coupling, which are the inherently small dimensions and light weight. Moreover, the manufacturing expenses of such a coupling, which are small where the coupling is constructed to transmit relatively low torque on the order of 20-30 cmkp, would become acceptably high if it were to be constructed for transmitting the much higher torque of commerical equipment, because the manufacture of the cooperating cylindrical surfaces on the components of the coupling would require a great accuracy in manufacturing to obtain the desired accuracy of response for the coupling.
Other types of safety couplings are already known from the art, wherein surfaces which are inclined in circumferential direction of rotation of the coupled components are pressed against one another by appropriate biasing means, in order to transmit torque. However, if such a coupling would be used to directly drive the tool spindle of a drill of the type in question, the biasing springs required to enable the coupling to transmit the torque levels in question would have to be so strong and heavy that they could no longer be accommodated within the rather confined area no longer be accommodated within the confined area of the housing of a handheld drill. Moreover, if the drill is of the hammer-drill variety, the use of a safety coupling associated directly with the tool spindle would be impossible for all practical purposes.
For this reason the prior art has proposed to use this second type of coupling in connection with an intermediate shaft which rotates at higher speed than the tool spindle and which transmits a correspondingly lower torque. This prior art construction utilizes inclined coupling surfaces which are constructed as the flanks of axial claws or projections and which are pressed together by means of at least one spring. However, drills, using this tyoe of coupling arrangement have been found to have the disadvantage that while the coupling disengages in the desired manner, there will -- depending upon the selected gear ratio -- a large moment that will act upon the tool spindle at the levl of the disengaging torque in the smaller gear ratio, whereas in the higher gear ratio there will be a smaller moment acting at the tool spindle at the level of the disengaging torque. In other words, depending upon which of the variable-gear ratios is selected, the coupling will disengage when the tool spindle is subjected to a higher or a lower torque, respectively. Evidently, this is disadvantageous and required improvement.